The invention relates to a local area network comprising a plurality of sub-networks and a plurality of bridge terminals, which bridge terminals are provided each for transmitting data between the terminals of at least two sub-networks via a wireless medium.
Such a local area network comprising a plurality of sub-networks is known from DE 196 37 026 A1. A sub-network contains a plurality of terminals which exchange via a radio transmission medium (wireless medium) cells generated in the asynchronous transfer mode. Data are transmitted between two sub-networks via a bridge terminal. How the transmission paths run through the bridge terminals is not further described in said document.
It is an object of the invention to provide a bridge terminal for connecting at least two sub-networks in a local area network, which bridge terminal makes unique transmission paths possible in the local area network.
The object of the invention is achieved by a local area network of the type defined in the opening paragraph
in that a bridge terminal, after being put into operation, is provided for transmitting a certain number of loop test messages to all the sub-networks connected to the bridge terminal, in that each bridge terminal is provided for transferring a loop test message received from a sub-network to all the other connected sub-networks and
in that the bridge terminal that has transmitted the certain number of loop test messages is provided for detecting that there is no loop when no loop test message returns after a predefined period of time.
According to the invention, a bridge terminal, after being put into operation, detects whether there is a loop in the local area network. If there is a loop in the local area network, one sub-network can receive the same data from a plurality of other sub-networks. Avoiding loops means that a sub-network can always receive data of another sub-network only from a specific bridge terminal and thus unique transmission paths are created.
The data, which are transmitted in the local area network, may have been generated in accordance with a packet transfer method. The packets may be transmitted as a whole, or as sub-packets via the wireless medium after further data have been added. A wireless medium is understood to mean a radio, infrared, ultrashell transmission etc. As a packet transfer method may be used, for example, the asynchronous transfer mode (ATM), which generates fixed-length packets which are called cells.
A bridge terminal which is put into operation first transmits a loop test message which contains, as shown in claim 4, an identification number for the bridge terminal transmitting the loop test message. A bridge terminal receiving a loop test message transfers this loop test message. When the bridge terminal that has transmitted a loop test message does not receive this message back after a predefined period of time, there is a loop. As described in claim 2, this loop test message may be transmitted a number of timesfor reasons of reliability. Random time intervals then occur between the respective transmission instants.
Claim 3 describes that a bridge terminal, once a loop has been detected, is not provided for transmitting data between the connected sub-networks.
In order not to load the local area network unnecessarily with false messages, a bridge terminal inserts a zeroed sub-net count into the loop test message. The evaluation of the sub-net count is defined in claim 5.
The invention further relates to a bridge terminal of a local area network including a plurality of sub-networks and further bridge terminals for transmitting data between the terminals of at least two sub-networks via a wireless medium.